This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In this study, we propose to use spectroscopic and polarimetric techniques to push the limits of optical intrinsic signal imaging (ISI) towards measuring the fast response of an illuminated cortex to a stimulus input. For this study, fast signals are defined as those that take place on timescales shorter than the hemodynamic response, which occurs on the order of seconds. Thus we propose to image light reflectance changes in the cortex that are due to electrical signals in the activated neurons (~10s of milliseconds) and to the transient increase in deoxygenated hemoglobin known as the initial dip (~100s of milliseconds). Such fast signals are crucial to understand because they are the least understood part of the brain response to a stimulus and are a direct measure of neural activity.